1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device, and more particularly, to a method of fabricating a complementary metal oxide silicon (CMOS) image sensor. The present invention relates to especially to a process after forming a pad from a metal line layer.
2. Description of the Related Art
Characteristics of a CMOS image sensor depend on sensitivity of a photo diode receiving external light particles. This sensitivity is mainly determined by a distance between a microlens and a photo diode, and also a thin film characteristic. Unlike a metal layer that completely reflects all light, a passivation nitride of a related art pixel block generally screens and reflects a relatively large portion of light compared to a passivation oxide. Accordingly, the sensitivity of a CMOS image sensor deteriorates.
On the other hand, a hydrogen anneal process, e.g., a sintering process in the presence of hydrogen or other reducing gas or gas mixture, is performed to improve characteristics of low illumination in a related art CMOS image sensor. Since a passivation nitride is applied thickly, it has a tendency to block hydrogen ions, thereby deteriorating process effectiveness.
Additionally, unlike a related art semiconductor manufacturing process, the hydrogen anneal process for a CMOS image sensor is performed after stacking the passivation nitride not after stacking the passivation oxide.
Disadvantages of a method of fabricating a related art CMOS image sensor are described with reference to FIGS. 1A through 1H. Especially, FIGS. 1A through 1H illustrate processes from forming a pad to fabricate a microlens until opening a pad.
Referring to FIG. 1A, after forming a pad 110, a passivation oxide 120 and passivation nitride 130 are formed. On the other hand, unlike an actual main chip region inside a wafer, an irregular wafer edge remaining layer 100 remains in an edge region of the wafer due to a wafer edge going through various photolithography processes. Because of the irregular wafer edge remaining layer 100, the adhesive strength weakens in an edge region of the wafer.
As illustrated in FIG. 1B, a hydrogen process is performed in a case of a CMOS image sensor. The hydrogen process is a thermal treatment process using gas that includes an appropriate ratio of nitrogen and oxygen gas. A wafer edge peeling 140 occurs during this thermal treatment process. In the wafer edge peeling 140, the passivation nitride 130 remaining in the wafer edge comes off in a circle form due to the deepening of an interlayer stress in the passivation nitride 130 or the rising of fluorine ion in a fluorinated silicate glass of the wafer edge remaining layer 100. Moreover, during the thermal treatment process, defect peeled from the wafer edge pops into a pixel region in an inner part of the wafer. A particle moved into the inner part of the wafer is called a circle defect 145.
Referring to FIG. 1C, scrubbing is performed to remove these circle defects 145, and the region of a pad 100 is opened through a photolithography process. Next, a cleaning and pad processing process including a pad ashing process, a solvent cleaning process, and a final cure process is performed. Although the scrubbing is performed to remove the circle defects 145, the circle defect 145 still remains.
Referring to FIG. 1D, a pad protective layer 150 is applied. The pad protective layer 150 may be a plasma enhanced tetra ethyl ortho silicate (PE-TEOS) layer or a thermosetting resin layer. The pad protective layer 150 has a thickness of 200 to 500  by a thin application, thereby making it easy for opening the pad 110 later.
FIG. 1E is a sectional view of a color filter array 160 by a color photolithography process. FIG. 1F is a sectional view of a planarization layer 170 by a planarization layer photolithography process. FIG. 1G is a sectional view of a convex microlens 180 by thermal reflow in a microlens process. Finally, FIG. 1H is a sectional view of opening the pad 110 by etching the pad protective layer 150 in a pad region. Next, it is possible to perform a probing test.